The continuous rise in the world population is accompanied by similarly constant increases in waste materials in industry, trades and households, the harmless disposal of which represents an ongoing problem, particularly when these waste materials contain constituents which are damaging to the environment or to health.
Waste materials of this kind are also generated by processes which are applied to reduce the waste materials, such as the incineration of domestic refuse or combustible wastes from industry; in refuse incineration, for example, 30 to 40% by weight of residues, which can contain toxic and/or environmentally damaging constituents, are obtained relative to the input quantity. Solid waste materials are also generated in the purification of flue gases, including those from refuse incineration plants, which are environmentally harmful because of their content of pollutants. The deposition of such waste materials which contain more or less water soluble constituents on unprotected dumps above ground is not possible, since the water from natural precipitation forms solutions with the water soluble constituents of such waste materials which permeate into the subsoil, damage it and represent a hazard for ground water. For these reasons, waste materials of this kind can only be deposited on aboveground dumps which are securely sealed off against the subsoil and surrounding land in the form of special refuse dumps and which are equipped with devices for the harmless disposal of liquids issuing from the dumped material.
Processes are known whereby waste materials in liquid or sludge form are converted into solid products through the admixture of flue ash and quicklime and of hot bitumen or meltable plastics in so-called silicate binders such as water-glass or of these binders themselves. Harmful substances can, however, also be released from these solid products by the action of moisture and precipitation if they are deposited on unprotected dumps, so that these solid products also can be stored only on special refuse dumps.
In DE-OS No.34 11 998 a process is proposed for the storage or ultimate disposal of solid, particulate waste materials in salt caverns equipped with pipelines for filling and evacuation. According to this process, these waste materials are first mixed with a rock salt solution withdrawn from a salt cavern to form a pumpable mixture which is then pumped into the salt cavern.
The process specification contained in DE-OS No.34 11 998 also admits the possibility of introducing particulate solids in a dry state into salt caverns through fall pipes. The particulate solids can also first be compactly enclosed in coating material and introduced in this form into the salt caverns by free fall through suitably dimensioned fall pipes.
In both cases, a corresponding volume of air is displaced from the salt cavern and this air is highly charged with the dust from the introduced waste materials and it must be purified by technically elaborate methods before being discharged into the atmosphere.
With the process described in DE-OS No.34 11 998, a volume of rock salt solution equal to the volume of introduced waste materials must be continuously withdrawn from the cavern and disposed of in a non-polluting manner. If no harmful substances originating from the waste materials have become dissolved in this rock salt solution, the rock salt solution can be dumped at sea without adverse effects, the only expense involved being that for transportation. If, however, the rock salt solution present in the salt cavern is expected to be contaminated with harmful materials, according to a recent, still unpublished proposed method, its density can be increased almost to the point of solidification by adding substances which react with binding of water of crystallization, and the salt cavern is then sealed. With this method, however, part of the volume of the salt cavern cannot be filled with waste materials.
The waste materials mentioned above are generated not only by production processes in industry, trades and in households, but also by processes applied to reduce the quantity of such waste materials, for example by incineration According to data presented by Thome-Kozmiensky in "Mullverbrennung und Rauchgasreinigung", Publ. E. Freitag, Berlin (1983), p. 1058, in 1978 6 million tons of domestic refuse were already being incinerated in 42 refuse incineration plants in the Federal Republic of Germany, from which however 2 million tons of combustion products were obtained which can contain environmentally damaging and even toxic constituents. On page 31 of the above publication it is stated that the residues from refuse incineration contain percentage by weight proportions of the following metals:
cadmium 11.7, PA1 chromium 1195, PA1 copper 871, PA1 lead 7273, PA1 zinc 26600, PA1 nickel 778.
These combustion products must therefore be treated as special refuse, the ultimate disposal of which cannot be effected on ordinary unprotected dumps.
Furthermore, in the purification of flue gases which is necessary in the incineration of domestic refuse to remove sulfur and nitrogen oxides and halogens, further quantities of waste materials are obtained which also contain heavy metals and which because of their contaminant content must be treated as special refuse.
By applying costly and elaborate measures, therefore, domestic refuse can be reduced to special refuse in refuse incineration plants in quantities representing about one third of the original input.
Desulfurization of flue gases of coal firing plants also generates considerable amounts of waste materials which consist of calcium, sulfate dihydrate (flue gas gypsum) or ammonium sulfate depending on the process employed. The quantity of flue gas gypsum generated in the Federal Republic of Germany is currently 2 million tons per year and is expected to increase in future. Providing that this flue gas gypsum is only contaminated with heavy metals within certain limits, it can be utilized for industrial purposes. For example, a process for the recovery of gypsum in flue gas desulfurization is known from DE-OS No.33 12 388.
It has been recommended that the ammonium sulfate obtained in flue gas desulfurization should be used as nitrogen fertilizer. Since however this ammonium sulfate is generally also contaminated with heavy metals, the introduction of excessive amounts of heavy metals into cultivated land may result. Ammonium sulfate is also generated in the manufacture of plastics and in the purification of coking plant gas. The total quantity of ammonium sulfate obtained from these known processes greatly exceeds the levels required for soil fertilization.
The depositing of such waste materials, which are more or less water soluble, on unprotected dumps above ground is impossible, since water from natural precipitation and from waste materials not dumped in the dry state form solutions with the water-soluble components of the deposited wastes, which permeate into the subsoil, and contaminate not only the subsoil but also the ground water. Only if aboveground dumps are securely sealed off from the subsoil and the surrounding land and if harmless disposal of the liquids issuing from the dumped materials is guaranteed, can waste materials of this kind be dumped in this way.
Waste materials of this kind can only be dumped in underground mine workings of abandoned mines which have not been sealed if they are contained in non-leaking containers.
Safe dumping of the waste materials referred to is thus technically always highly demanding.
It is also conceivable to dump solid waste materials in salt caverns. Without additional measures, however, such waste materials can only be trickled into the salt cavern in superfine particulate form and in a completely dry state through fall pipes. The air which is thereby displaced from the salt cavern must, however, be conducted to a de-dusting plant in order to reliably remove from it the dust originating from the waste material. The same applies for pneumatic introduction of the particulate solid waste materials into a salt cavern. These methods of dumping solid waste materials in salt caverns, which are of only limited practicability, could only be realized with the aid of technically very elaborate systems.
It is naturally also feasible to enclose the particulate waste materials, divided into partial quantities, in wrappings consisting, for example, of plastic film and to introduce them into the salt caverns in this form by free fall through suitably dimensioned fall pipes. Apart from the technical demands involved in filling the waste materials in the wrappings and ensuring that they are tightly sealed, this method requires the use of fall pipes whose internal diameter must be greater than that of the piping generally used for salt caverns. With this procedure, moreover, considerable dust loading of the air displaced and issuing from the salt cavern is not excluded, since the wrappings may burst during their free fall through the fall pipes and when hitting the bottom of the salt cavern or the waste materials which have already been introduced in wrappings. This method therefore also necessitates technically demanding purification of the air issuing from the salt cavern.
A process is also known from DE-OS Nos.31 41 884 and 31 41 885 whereby pumpable waste materials are introduced into salt caverns for ultimate storage or for complete or partial re-use. With this method, liquid waste materials are mixed with solid waste materials to form pumpable mixtures which are introduced into salt caverns for ultimate disposal or disposal in such a way that re-usable parts of the stored material can be withdrawn again from the salt cavern. To ensure the pumpability of these mixtures, their content of liquid waste material must be relatively high, so that this method therefore allows only relatively limited quantities of solid waste materials to be introduced into a salt cavern, the remaining volume of which is filled with liquid.